Rename column slotsync_skip_at to slotsync_last_skip.

Rename column slotsync_skip_at to slotsync_last_skip.

Commit 76b78721ca introduced two new columns in pg_stat_replication_slots
to improve monitoring of slot synchronization. One of these columns was
named slotsync_skip_at, which is inconsistent with the naming convention
used for similar columns in other system views.

Columns that store timestamps of the most recent event typically use the
'last_' in the column name (e.g., last_autovacuum, checksum_last_failure).
Renaming slotsync_skip_at to slotsync_last_skip aligns with this pattern,
making the purpose of the column clearer and improving overall consistency
across the views.

Author: Shlok Kyal <[email protected]>
Reviewed-by: Michael Banck <[email protected]>
Discussion: https://postgr.es/m/20251128091552[email protected];lightning.p46.dedyn.io
Discussion: https://postgr.es/m/CAE9k0PkhfKrTEAsGz4DjOhEj1nQ+hbQVfvWUxNacD38ibW3a1g@mail.gmail.com